The Engine Control Moduel (ECM) has an on board diagnostic system, which detects malfunctions related to engine sensors or actuators.
The ECM also records various emission-related diagnostic information
The malfunction indicator lamp (MIL) on the instrument panel lights up when the same malfunction is detected in two consecutive trips (Two trip detection logic), or when the ECM enters fail-safe mode.

Instructions

The first step to read the codes is setting the ECM in Diagnostic Test Mode II. Once the ECM is in Diagnostic Test Mode II, the "Service Engine Soon"
warning light will start flashing. Count to flashes to determine the code

A particular trouble code can be identified by the number of four-digit numeral flashes. The 'zero' is indicated by the number of ten flashes. The length of time the 1,000th-digit numeral flashes on and
off is 1.2 seconds consisting of an ON (0.6-second) - OFF (0.6-second) cycle. The 100th-digit numeral and lower digit numerals consist of a 0.3-second ON and 0.3-second OFF cycle. A change from one digit
numeral to another occurs at an interval of 1.0-second OFF. In other words, the later numeral appears on the display 1.3 seconds after the former numeral has disappeared. A change from one trouble code to
another occurs at an interval of 1.8-second OFF. In this way, all the detected malfunctions are classified by their DTC numbers. The DTC 0000 refers to no malfunction.

How to read the engine codes without a Nissan CONSULT or scan tool. (Set ECM in Diagnostic Test Mode II)